The present invention relates to an image producing method for reconstructing an image by Fan-Para converting and back projecting radiation projection data, an image producing apparatus and a radiation tomographic imaging apparatus, and a program therefor.
In the third generation X-ray CT (Computed Tomography) apparatus, a fan-beam X-ray is used for collecting projection data. Also, there is a case where an image is reconstructed by performing back projection processing after converting the fan-beam projection data thus collected into parallel-beam projection data.
Generally, the above conversion is called “Fan-Para conversion,” and the image reconstructing method using the Fan-Para conversion is called a “Fan-Para conversion method.” The Fan-Para conversion method is applied mainly for the purpose of securing evenness of CT values, suppressing an artifact when performing MPR (Multi-Plane Reconstruction), and reducing the calculation amount, etc.
Incidentally, in the Fan-Para conversion method, Fourier transform which is frequency conversion, is performed on the projection data after the Fan-Para conversion. Therefore, the projection data after the Fan-Para conversion must be regular-interval parallel-beam projection data in which radiation paths corresponding to data of respective channels are correctly parallel to each other and are arranged at regular intervals in a channel direction (see, for example, paragraph [0004] etc. of Japanese Patent Application Laid-open No. 2012-005757).
Therefore, it is often the case in Fan-Para conversion that relatively high-precision regular-interval parallel-beam projection data is acquired by applying interpolation processing in a view direction, rearrangement processing, and interpolation processing in a channel direction to actually measured fan-beam projection data.
As compared with the data before the interpolation processing, however, the regular-interval parallel-beam projection data acquired by the interpolation processing contain more errors being different from the true value, which brings about degradation in spatial resolution in the reconstructed image. In particular, the degradation in spatial resolution grows more prominently as they become distant from the center, i.e., an iso-center, of a scan field of view region.
Under such circumstances, there is a demand for a technique capable of suppressing the degradation in spatial resolution of the reconstructed image even when the Fan-Para conversion is performed.